In marine seismic exploration, a source of acoustic energy is released into the water periodically to produce appropriate sonic pulses or shock waves that propagate into the earth's surface beneath the water. These pulses propagate down through the water, across the marine floor, into the subfloor geologic formations and are reflected back as acoustic waves. An array of geophones, hydrophones or like equipment detect the reflected acoustic waves and convert such waves to electronic signals. These electronic signals are recorded for interpretation of the subsurface formation. Analysis of these electronic signals can provide an indication of the structure of the subfloor geological formation and attendant petroleum accumulation in those formations.
The term "water" as used herein is meant to include swamp water, mud, marsh water and any other liquid containing sufficient water to enable operation of the invention.
In one method of onshore seismic exploration, an underwater acoustic energy source is placed in a shallow water pit or mud pit used otherwise for routine drilling operations. Reflected sonic pulses generated by the underwater source may be detected by geophones placed on the ground. Alternately, geophones may be lowered down a nearby well. Reflected sonic pulses may also be detected by hydrophones in a water or mud pit.
There are many ways of generating a sonic pulse or wave in a liquid. For instance, explosives introduce strong pulses into the water and accordingly achieve substantial penetration into subfloor formations. Certain obvious drawbacks exist in their use. They can be dangerous to store, handle, and use. When used in open water they harm some marine life forms. In crowded areas, such as harbors, explosives cannot be used at all. Explosives are also expensive to use. Modification of the explosive source's sonic frequency spectrum of "signature" to achieve an acceptable spectrum distribution is difficult.
Another method of generating a sonic pulse is by discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing implosive gaseous bubble. This method of sonic pulse generation is commonly used when high resolution response from near-surface geologic formations is desired. However, the efficiency of this method is low and only a small percentage of the energy charged to the capacitors is typically found in a shock wave produced on discharge.
Apparatuses using explosive gases (such as mixtures of propane and air or mixtures of propane and oxygen) to produce a sonic pulse on ignition have gained wide acceptance. The two major types of explosive gas guns are (1) those which operate by exploding a gas mixture behind a flexible membrane which in turn is in contact with the water and (2) those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Pat. No. 3,658,149. An example of the latter can be found in U.S. Pat. No. 4,193,472.
Acoustic energy sources using high pressure compressed gases instead of an explosive mixture have also achieved a wide acceptance in the industry. Typical designs for open ported compressed gas guns are found in U.S. Pat. No. 3,653,460 to Chelminski and U.S. Pat. No. 4,141,431 to Baird. A typical compressed gas gun for marine seismic exploration comprises a housing which contains a chamber adapted to confine a charge of compressed gas at high pressure. The chamber is fitted with a valve. The valve is closed while the pressure is built-up in the chamber. When the gun is "fired", the valve is rapidly opened. This allows the compressed gas to expand out of the chamber and through exhaust ports in the housing into the surrounding medium to create an acoustic pulse.
Recently, a particular compressed gas gun, the air gun, has been a major marine seismic energy source. The typical air gun is of a configuration described above wherein the compressed high pressure gas is air. Currently, the compressed air in such guns is maintained at pressures between 2,000 and 6,000 psi prior to release in the water to create the desired acoustic wave.
State of the art air guns normally comprise a cylindrical housing containing symmetrically distributed exhaust ports through which the compressed gas is released when a valve is opened in the gun. The exhaust port configuration of these underwater compressed air guns may vary. In a common configuration, four exhaust ports are symmetrically distributed around the periphery of the cylindrical housing of the compressed gas gun. PAR.RTM. Air Guns available from Bolt Technology Inc., Norwalk, Conn. are examples of air guns with four symmetrically distributed exhaust ports. In another configuration, compressed air is released through one 360.degree. exhaust port about the periphery of the compressed gas gun. The external sleeve air gun designed by Geophysical Services Inc. of Dallas, Tex. is an example of an air gun with one 360.degree. exhaust port. In an external sleeve air gun, a shuttle valve concentric with the gun housing slides along the outer surface of the housing to open and close a 360.degree. exhaust port.
When a compressed gas gun is operated as a seismic source near the atmosphere-water interface, the bubble caused by the release of compressed gas can "blow-out". An above-surface "blow-out" occurs when a substantial portion of the released gas bubble expands into the atmosphere above the interface. These blow-outs can be partial or complete. When the bubble blows out, most of the gun's low frequency acoustic energy is lost to the atmosphere, rendering the source less desirable for deep seismic exploration.
As earlier discussed, common compressed gas guns (especially air guns) typically utilize several exhaust ports symmetrically spaced about the outer periphery of the gun body. Operation of a gun positioned horizontally will cause at least one of the ports to face the atmosphere-water interface. If the gun is operated in shallow water (usually less than ten feet), and is substantially charged, a partial or complete blow-out may occur from this near-surface port. This will cause the loss of acoustic energy rendering the seismic source inefficient for exploration.